This invention relates to the manufacturing of novel calcium silicate hydrate (xe2x80x9cCSHxe2x80x9d) crystalline structures, and to pigment products, and novel to paper products produced therewith.
The paper industry currently utilizes many different types of fillers as a substitute for pulp fiber, as well as to provide desired functional and end-use properties to various paper and paper products. For example, clay has long been used as a filler or fiber substitute. Importantly, the use of clay also provides an improvement in print quality. However, one disadvantage of clay is that it is relatively low in brightness. And, the use of clay in papermaking leads to a decrease in tensile strength of the paper sheet, and to reductions in paper sheet caliper and stiffness.
Calcined clay was introduced to the paper industry in an effort to improve brightness and opacity in paper. However, one significant economic limitation of calcined clay is that it is relatively expensive. Also, physically, calcined claim is highly abrasive.
Titanium dioxide, TiO2, is another example of a filler commonly used in papermaking. Most commonly, titanium dioxide is used to improve opacity of the paper sheet, and, in some cases, it is used to improve sheet brightness as well. Use of titanium dioxide is limited, though, because it is extremely expensive. Unfortunately, it is also the most abrasive pigment on the market today. This is important because highly abrasive pigments are detrimental in the paper industry since they wear down critical paper machine components, such as forming wires, printing press plates, and the like, ultimately leading to high life cycle costs due to the constant repair and maintenance costs.
When calcined clay was first introduced, it was touted as a titanium dioxide extender. Although it did succeed in extending TiO2, it is nonetheless abrasive, and it is more expensive than either standard clay or market pulp fiber.
More recently, and particularly since the mid 1980""s, ground calcium carbonate(GCC) has been used as a low cost alkaline filler. Although GCC improved sheet brightness, one downside to GCC was that it too is abrasive. Moreover, use of GCC reduces tensile strength, caliper and stiffness of paper sheets. Consequently, a paper sheet containing GCC tends to be rather limp.
Finally, one of the most commonly used alkaline paper fillers is precipitated calcium carbonate(PCC). PCC is presently one of the best compromise solutions for providing a high brightness filler at an economically feasible price. However, a significant downside to the use of PCC in paper sheets is that PCC provides a lower light scattering power than either TiO2 or calcined clay. Also, it often reduces sheet strength and stiffness.
Thus, the paper industry still has an unmet need, and continues to look for, a multi-functional pigment that can simultaneously provide two or more of the following attributes:
a) cost that are less than TiO2;
b) better optical properties than calcined clay;
c) better optical properties than GCC;
d) better optical properties than PCC;
e) minimal tensile strength loss associated with increased filler usage;
f) at least some improved strength characteristics, such as sheet stiffness.
In addition to the just stated criteria, if a paper filler could also simultaneously improve sheet porosity (i.e., provide a more closed sheet) yet provide higher sheet caliper, it would be a very highly desired filler material. To date, no single paper filler with such attributes has been brought to the market. Consequently, the development and commercial availability of such a filler would be extremely desirable.
Finally, the current industry demand for printing papers, especially the rapidly increasing demand for ink jet paper, requires a high performance paper. The performance of such paper would be enhanced by the availability of a pigment that would provide excellent water and oil absorption capacities, so that the paper could quickly capture and prevent ink from spreading or bleeding, as well as aid in surface drying of the ink.
Some of the key requirements for an ideal papermaking pigment can be summarized as set forth in Tables 1, 2 and 3 below.
Currently, the papermaking industry uses various combinations of available fillers in order to optimize the properties as may be desired in a particular papermaking application. However, because currently available fillers reduce sheet strength to at least some extent, the industry relies on strength additives, such as starch and/or polymers, to maintain the desired paper strength properties when fillers are utilized. Unfortunately, because different pigments have different particle charge characteristics, additions of multiple pigments and additives in the paper making system often create an extremely complicated chemical system which may be somewhat sensitive and difficult to control.
In summary, there remains a significant and as yet unmet need for a high quality, cost effective filler which can be used to simultaneously achieve desired optical properties and sheet strength in paper products. Further, there remains a continuing, unmet need for a method to reliably produce such a pigment which has desirable optical properties and which provides significant cost benefits when compared to the use of titanium dioxide or other pigments currently utilized in the production of paper.
Accordingly, an important objective of my invention is to provide a process for the manufacture of unique calcium silicate hydrate (xe2x80x9cCSHxe2x80x9d) products, which provide crystalline structures with desired brightness, opacity, and other optical properties.
Another important and related objective is to provide an economical substitute for current paper fillers such as titanium dioxide.
A related and important objective is to provide a method for the production of novel paper products using my unique calcium silicate hydrate product.
An important objective is to provide a new calcium silicate hydrate product with low bulk density, good chemical stability (particularly in aqueous solutions), and a high adsorptive capability, among other properties.
These and other advantages, and novel features of my multi-phase calcium silicate hydrates, the method for their preparation, and the improved pigments and paper products produced therewith will become evident and more fully appreciated from full evaluation and consideration of the following detailed description, as well as the accompanying tables and drawing figures.
I have now discovered the process conditions required to reliably produce unique calcium silicate hydrate products with particularly advantageous properties for use as a filler in papermaking. The products are produced by reacting, under hydrothermal conditions, a slurry of burned lime (quick lime) and a slurry of fluxed calcined diatomaceous earth (or other appropriate starting siliceous material). Preferably, a fine slurry of each of the lime and the fluxed silica are utilized.
For one of my CSH products, the lime slurry is prepared by providing about 1.54 pounds of suspended solids per gallon of lime slurry. The silica slurry is prepared by providing about 1.55 pounds of suspended solids per gallon of water. The slaking of the lime slurry raises the temperature of the slurry to near the boiling point; this is accomplished before adding the same to the fluxed silica. The slurry of fluxed calcined diatomaceous earth is heated to near the boiling point, also, before it is mixed with the lime slurry. When both slurries are near atmospheric boiling point conditions, then they are mixed together and stirred, while being retained under pressure in an autoclave or similar reactor. Temperature of the reaction slurry is raised to between about 245 C. and 260 C., and the reaction is continued for about two hours, more or less. The CaO/SiO2 ratio is maintained, in the feed materials, of about 1.35 (+/xe2x88x92 about 0.10) moles CaO to 1 mole of SiO2. After the reaction is completed, the product is cooled before the pressure is released and the product crystals are harvested.
Generally, the product of the above described reaction is a multi-phase mixture (i.e., two different forms or phases are present in the product), predominantly of foshagite, with some xonotlite. Importantly, small, haystack like particles containing complex multi-phase crystalline optical fibers are produced that can be advantageously employed in papermaking for coating and for wet end fillers. However, the hydrothermally produced multi-phase crystalline optical fibers are vastly improved over previously produced hydrothermal calcium silicate hydrates of which I am aware, at least with respect to their physical properties, their optical properties, and their utility as a filler in papermaking. Moreover, my unique CSH products are suitable for multiple end uses, such as filler for value added papers, for commodity papers, for newsprint, paper coating applications, as well as for paints, rubber compositions, and other structural materials.
It is important to appreciate that my hydrothermal process for the manufacture of my unique multiple phase calcium silicate hydrates (xe2x80x9cCSH""sxe2x80x9d), including my novel multi-phase mixture of foshagite and xonotlite, (CaO4(SiO3)(OH)2 and C6Si6O17(OH)2, respectively), results in a unique mixture of calcium silicate hydrates which have a unique and distinct X-ray diffraction pattern.
Further, the variables that affect the chemical composition of my CSH products, and the primary and secondary structure of the CSH particles and their characteristic properties, can be affected, among other things, by (a) the CaO/SiO2 mole ratio, by (b) concentration of the CaO and of the SiO2 in the reaction slurry, (c) the reaction temperature, and (d) the reaction time. By manipulating the just mentioned variables, I have been able to develop two novel pigment products. Those two products can be generally described as follows:
(1) A multi-phase calcium silicate hydrate having a primary phase of foshagite, and a secondary phase of xonotlite. I refer to this product as xe2x80x9cTiSilxe2x80x9d brand calcium silicate hydrate.
(2) A multi-phase calcium silicate hydrate complex having a primary phase fraction of riversidite with a minor phase fraction of xonotlite. I refer to this product as xe2x80x9cStiSilxe2x80x9d brand calcium silicate hydrate.
The first product is formed with a high CaO to SiO2 mole ratio (about a 1 to 1, to about a 1.7 to 1 ratio of CaO to SiO2), at a high temperature (xcx9c200xc2x0 C.-300xc2x0 C.), with a low final slurry concentration (xcx9c0.4-0.6 lb of solids per gallon of slurry), and with a reaction time of approximately 2 hours. It has a characteristic X-ray diffraction pattern as shown in FIG. 1. The scanning electron micrographs (SEMS) of this product are shown in FIGS. 2 and 3. As is evident from the SEMs, this product consists of primary, fibrous particles joined together, and thus, produces a secondary, three dimensional, xe2x80x9chay-stackxe2x80x9d structure. The physio-chemical characteristics of this product are unique. For example, extremely high water absorption is provided. This pigment also provides unique paper properties when utilized in papermaking. For example, this pigment, when used as a filler, can improve the optical properties along with sheet strength, sheet bulk, sheet smoothness, and sheet porosity, simultaneously.
The second product is formed by reacting lime and silica with a low mole-ratio (about a 0.85 to 1 ratio of CaO to SiO2), a low reaction temperature (xcx9c180xc2x0 C. to 190xc2x0 C.), at a high final slurry concentration (xcx9c0.7-1.0 pounds of solids per gallon of slurry), and with a reaction time of approximately 2 hours. This calcium silicate is quite different from the first product just mentioned above and its unique X-ray diffraction pattern is given in FIG. 4. The scanning electron micrographs (SEMS) for this product are given in FIGS. 5 and 6. As the SEMs indicate, this product consists of some fibrous growths that in turn grow randomly and almost continuously to provide an irregular globular structure. This product is uniquely formulated to provide ultra high sheet stiffness when it is used as a filler in paper.
In summary, the unique features of these hydrothermally produced calcium silicate hydrate products include:
a unique crystallo-chemical composition
a multi-phase crystal system
a primary and secondary fibrous particle structure
a high water absorptivity (in the xcx9c300%-1000% range).
The result of the unique properties and physical structure enable these unique CSH products to provide a combination of beneficial properties to paper products in a manner heretofore unknown by paper fillers. For example, the use of these products in paper can increase sheet bulk and Gurley porosity, simultaneously. In addition, these products are made up of large particles, but the products can still scatter light better than PCC, GCC, clay, or even calcined clay.
In order to prepare my unique calcium silicate hydrates (CSH) products, it is first necessary to prepare a source of calcium. This is normally accomplished by the formation of a slurry of calcious material, most commonly lime however, there are several different sources of calcium, which may be used. Some examples are CaCO3, CaCl2, and hydrated lime. I have found it advantageous to employ pebble lime, if less than xc2xd inch dimension. First, the CaO was slaked in water. The amount and the rate of addition of lime were set and maintained in order to obtain a desired concentration of lime slurry. Because the slaking of lime is an exothermic process, it was necessary to control both the rate of addition of lime and the quantity of water used. When slaking, the best temperature was determined to be near boiling, i.e., close to 100xc2x0 C. (212xc2x0 F.) in order to form lime particles as fine as possible. Once the slaking was complete, the lime slurry was then screened through a 200 mesh screen to remove any grit and oversized particles. The screened and slaked lime slurry was tested for available lime (as CaO) and then transferred to an autoclave.
The chemistry of the slaking process can be given as follows:
CaO+H2Oxe2x86x92Ca(OH)2xe2x80x83xe2x80x831)
(solid) (aqueous)
Ca(OH)⇄Ca+++2OHxe2x88x92xe2x80x83xe2x80x832)
(aqueous)
The solubility of calcium hydroxide slurry is inversely proportional to the temperature, as indicated in FIG. 7.
Next, it is necessary to prepare a slurry of siliceous material (i.e., a SiO2 slurry). Various siliceous materials such as quartz, water glass, clay, pure silica, natural silica (sand), diatomaceous earth, fluxed calcined diatomaceous earth, or any combination thereof may be utilized as a source of siliceous material. I prefer to utilize an ultra fine grade of fluxed, calcined diatomaceous earth. This raw material was prepared into a slurry of xcx9c1.55 lbs of solids per gallon water. The slurry was then preheated to near boiling, i.e., near 100 C.
Importantly, the solubility of silica (unlike that of Ca(OH)2), is directly proportional to temperature, as seen in FIG. 8. For example, quartz (line A in FIG. 8) is only slightly soluble up to 100xc2x0 C. From 100xc2x0 C. to 130xc2x0 C., it starts solubilizing and around 270xc2x0 C., it reaches its maximum solubility of about 0.07%.
The dissolution of silica can be represented as follows:
(SiO2)n+2n(H2O)xe2x86x92nSi(OH)4xe2x80x83xe2x80x833)
The solubility of silica can be increased by raising the pH, and/or by using various additives (i.e. sodium hydroxide). In addition the rate of silica solubility is also a function of particle size, thus to enhance solubilization of the silica, I prefer to utilize ultra fine fluxed calcined diatomaceous earth.
Next, the siliceous slurry was mixed with the lime slurry in an autoclave, to achieve a hydrothermal reaction of the two slurries. Important, the amount of CaO in the lime slurry and the amount of SiO2 in the fluxed calcined diatomaceous earth slurry were pre-selected to provide a predetermined CaO/SiO2 mole ratio. Also, the concentration of the two slurries (CaO and SiO2) was selected so that the final concentration of the reaction mixture in the autoclave falls between about 0.2 pounds of solid per gallon of slurry to about 1.0 pounds of solid per gallon of slurry.
The hydrothermal reaction itself was carried out in a pressurized vessel, with three major steps:
1) Heating the slurry to the desired temperature (e.g. 180xc2x0 C. to 300xc2x0 C.)
2) Reacting at temperature for a specified time (e.g. 60 min to 240 min).
3) Stopping the reaction and cooling down
In my laboratory, the reaction autoclave was cooled by passing quenching water through an internal cooling coil, or by utilizing an external jacketed cooling system. I prefer to utilize a cool down process of from approximately 25 to 30 minutes to drop the temperature from about 230xc2x0 C. to about 80xc2x0 C., as indicated in FIG. 9.
The process steps just mentioned are very important. This is because I have utilized the inverse solubilities of lime and silica with respect to temperature and time in an effort to produce the desired reaction composition, to arrive at the desired multi-phase calcium silicate hydrate product.
Without limiting my invention to any particular theory, I can postulate the following reaction during the hydro-thermal reaction between calcious material and siliceous material. First, during the heating process, very few free Ca++ ions are available. After 100xc2x0 C., the silica starts going into a gel stage. Beyond 130xc2x0 C., the silica ions become available for reacting. As the temperature nears 180xc2x0 C., the calcium ion Ca++ reacts with the Si+ ion to form a metal silicate. The reaction can be written as follows:
x[Ca+++2OHxe2x88x92]+y[Si(OH)4]xe2x86x92CaOx(SiO2)y+(x+y)H2Oxe2x80x83xe2x80x834)
Where: x=1 to 6
y=1 to 6
The solid Ca(OH)2 particles react with SiO2 in the gel phase to give a calcium silicate hydroxide whose crystallo-chemical structure can be written as Ca6Si6O17(OH)2 (Xonotlite). As the temperature is further raised from 180xc2x0 C. to 250xc2x0 C., calcium silicate hydride condenses with the remaining Ca(OH)2 particles to give yet another calcium silicate hydroxide, this time with a distinct X-ray diffraction pattern and a crystallo-chemical formula of CaO4(SiO3)3(OH)2 (Foshagite).
Further, I have developed my hydrothermal reaction process so that more than one unique calcium silicate hydrate can be produced. In this respect, it is important to note that the following variables are critical in producing a desired end product:
1) Slaking Temperature
2) CaO/SiO2 mole ratio
3) Slurry Concentration
4) Reaction Temperature
5) Reaction Time at Temperature
By changing these variables, a product having several different phases of calcium silicate hydroxide can be produced. Some of these phases may include:
Although not normally important, one should note that my final product CSH composition may also contain minor amounts of calcitexe2x80x94aragonite, produced as a result of side reactions.
The first and most important product of my process is a multi-phase CSH composition having various amounts of phases of matter represented by CaO4(SiO3)3(OH)2 (Foshagite) and Ca6Si6O17(OH)2 (Xonotlite). A unique X-ray diffraction pattern for this product is provided in FIG. 1. In that XRD, the crystallochemical formula of the mixture, and the characteristic d spacings, are given below:
The Scanning Electron Micrographs (SEMs) representing this first product are provided in FIGS. 2 and 3. As shown in FIGS. 2 and 3, it is important to note that the product consists of primary particles and secondary particles. The primary particles have a diameter between 0.1 and 0.2 microns and a length between 1.0 and 4.0 microns. FIG. 3 also indicates that the primary particle has two phases. The rod or ribbon like structure is characteristic of xonotlite (Ca6Si6O17(OH2)) while the predominant structures are thin and fibrous, characteristic of foshagite (Ca4(SiO3)3(OH)2). The diameter of the foshagite crystals ranges from 0.1 to 0.3 microns and the length is ranges from 2.0 to 5.0 microns.
The SEM of FIG. 3 reveals a secondary, three dimensional structure. This three dimensional structure is believed to be formed by the interlocking of the fibrous material and the continuous growth of the xe2x80x9cgelxe2x80x9d like material at the intersection of the individual particles. This may also be the reason that the secondary structure is fairly stable. Importantly, the secondary structure can generally withstand the shear forces encountered during the discharge of material from pressure vessels after the reaction has completed, as well as shear forces encountered during papermaking. This is seen, for example, in that the secondary structure maintains its xe2x80x9cbulk densityxe2x80x9d during some of the end use processes such as calendering during paper making. The particle size of secondary structure, as measured by particle size measuring devices like the Malvern Mastersizer, is in the range of 10-40 microns.
The calcium silicate hydroxide mixture of my invention also has very high brightness characteristics. A comparison with other pigments is given below:
Various pigments and their typical published brightness values are as follows:
One of the most significant characteristics of the composition of matter produced by my process is the ability of these multiple phase calcium silicates to absorb large amounts of water. These calcium silicates can adsorb anywhere from 350% to 1000% of their weight. This high water absorption capacity makes my pigment extremely well suited for preventing ink strike through in writing and printing papers, newsprint and more.